Field of Science

Enigmatic Triassic Fossils - Agnosphitys cromhallensis

Alton Dooley over at Updates from the VPL has some excellent photos of the holotype material of Agnosphitys cromhallensis from the Upper Triassic fissure fills of England. The name, 'unknown begetter', reflects upon the unclear phylogentic relationships of this taxon. The original describers, Fraser et al. (2002), considered it to be a dinosauromorph more derived than Eoraptor and Herrerasaurus, but just outside of Dinosauria. It is known from an isolated left ilium, a left maxilla, a right humerus and a left astragalus. There is also a referred tooth. The association of this material was questioned with Langer (2004) considering it a nomen dubium.

The ilium possesses a distinct brevis shelf and 'open' ventral portion of the acetabulum (so it was slightly perforated), yet only has scars for two sacrals. The subrectangular deltopectoral crest on the humerus is typical for dinosauromorphs and the astragalus is very similar to that of basal saurischians (Nesbitt et al., 2007). Thus these specimens contain a melange of basal and derived characters making its phylogenetic placement uncertain (Fraser et al., 2002). This is also a very small animal as the humerus length is under 35mm. Unfortunately, more material needs to be found to elucidate the relationships of this animal, but what is present is not only very well preserved but extremely interesting.

REFERENCES

Fraser, N. C., Padian, K., Walkden, G. M., and Davis, A. L. M. 2002. Basal dinosauriform remains from Britain and the diagnosis of the Dinosauria. Palaeontology 45:79–95.

Langer, M.C. 2004. Basal Saurischia. In: Weishampel, D.B., Dodson, P., & Osmolska, H. (Eds.). The Dinosauria (2nd Edition). Berkeley: University of California Press. Pp. 25–46.

Nesbitt, S. J., Irmis, R. B. & Parker, W. G. 2007. A critical re-evaluation of the Late Triassic dinosaur taxa of North America. Journal of Systematic Palaeontology 5, 209-243.

6 comments:

  1. A partial repeat of what I wrote on the original blog-

    The phylogeny in Fraser et al. (2002) was outdated, since basically every other analysis since Herrerasaurus was redescribed by Sereno and Novas in 1993 placed that taxon in Dinosauria and Saurischia. Herrerasaurus, Eoraptor and Staurikosaurus are now known to have had three sacral vertebrae after all (Sereno, 2007; Bittencourt and Kellner, 2009), making the number in the disarticulated Agnosphitys questionable. Its well developed brevis fossa would merely exclude it from Herrerasauridae. The only two authors to include Agnosphitys in phylogenetic analyses so far found it to be a basal theropod (Yates, 2007) or a guaibasaurid sauropodomorph (Ezcurra and Novas, 2009).

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  2. My gut (and the general morphology of the ilium) would tell me that the Ezcurra and Novas (2009) is the best hypothesis right now, but the material is pretty scrappy. The sacral vertebra question probably won't be solved without more material.

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  3. But all this assumes that the material can be positively assigned to one taxon...

    I do favor a saurischian placement for the material though, just not sure if there is enough material to support a sauropodomorph placement...

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  4. There are actually a few additional ilia from the type locality in our collection that are labeled Agnosphitys (which means that's what Nick thought they were), but I don't think they clarify the sacral issue. On the other hand, we have a very large amount of unprocessed material from the type locality, so there is some hope for additional material. But pretty much everything in the fissure fills seems to be disarticulated.

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  5. Cool photographs, thanks for the link Bill. :]

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  6. And thanks also to Alton for posting those pictures in the first place. ;]

    Nick

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